Designing an anti-Ruddlesden-Popper Polar Metal

The combination of metallic conductivity and broken inversion symmetry found in polar metals allows for several diverse and desirable properties. Recent interest has led to many predicted materials, but relatively few have been experimentally synthesized. Even fewer synthesized compounds exhibit a symmetry-lowering phase transition, limiting potential applications as well as opportunities to experimentally probe structure-property relationships. In the pursuit of designing a new class of distortive polar metals, we use the anti-Ruddlesden-Popper (ARP) structure type (chemical formula A₄B₂O) as a platform to explore new potential polar metals. We identify two design strategies: (1) swapping the A-site of known ARP ferroelectrics with transition metals to induce metallicity and (2) constructing n=1 layered analogs of known polar metal anti-perovskites. Testing a suite of 19 candidate structures using first principles simulations, we identify multiple dynamically stable potential ground states in the Cm space group along with a favorable predicted enthalpy of decomposition. We believe this work provides a template for continued expansion of the number and variety of polar metal classes and propose design rules for further development of the ARP polar metal family.